Since ceramics, high-melting point metals and ceramics/metal composites are generally produced by sintering raw material powders, shape assignment to product is performed mainly at the time of molding. However, the easiness of the shape assignment depends on molding method. For example, a disk product can be easily molded by mold pressing method. However, a product with complicated shape requires molding of a massy molded body by CIP (Cold Isostatic Press) before shape assignment by machining.
Gel cast molding is a method for obtaining an inorganic powder molded body by solidifying liquid slurry containing an inorganic powder by mutual chemical reaction of organic compounds contained in the slurry. This process is excellent in accurate shape assignment property because of accurate transferability of mold shape. However, in the case of a product having a closed structure, this process is not applicable because of the non-releasable shape, or requires separate providing of a core mold as in lost-wax process for assignment of inner surface shape.
Particularly, in a product having an end hole diameter smaller than a body part inside diameter such as a metal halide lamp light emitting tube or a high-pressure sodium lamp light emitting tube, it is difficult to improve the productivity. It is considerable to divide each element constituting the light emitting tube to simply-shaped small parts and obtain each small part by extrusion molding, dry back press molding, or mold press molding. In this case, a plurality of parts can be integrated at the time of sintering using the sintering shrinkage difference between them. Further, a method for obtaining a preliminarily integrated molded body by a gel cast molding comprising separately molding a core and injecting slurry between the core and an outer mold is also adopted (refer to Republished Patent Applications WO 2002-085590A1 and WO 2005-028170A1).
To separately mold a plurality of parts and join them to an integrated unit, a method described in Japanese Patent Publication No. 2004-519820 is further adoptable. In this joining method, an organic binder is included in each ceramic body, and the organic binder is locally melted by simultaneously heating a joint surface of a first ceramic body and a joint surface of a second ceramic body. The first and second joint surfaces are then brought into contact with each other through the locally melted binder, and compression and elongation are alternately provided to the boundary surface area between the two joint surfaces, whereby joining parts are integrated together.